JOHANNESBURG—With their former president in the hospital and their nation's economic promise unfulfilled, South Africans are suffering from a powerful moment of Nelson Mandela nostalgia.

On Wednesday, President Jacob Zuma elicited rare across-the-aisle cheers from parliament when he said the 94-year-old statesman was responding to treatment after five days in a Pretoria hospital for a lung infection. "We are very happy with the...